On Thursday, North Korea launched two short-range missiles that flew 500 km before splashing into the sea.

"Dear comrade Kim Jong Un said work ... must be strengthened to improve nuclear attack capability and issued combat tasks to continue nuclear explosion tests to assess the power of newly developed nuclear warheads and tests to improve nuclear attack capability," KCNA reported on Friday.

The statement is the latest in a series of provocative actions and posturing North Korea adopted after the UN imposed the toughest sanctions on it for its nuclear tests and ballistic missile programme.

On Thursday, North Korea launched two short-range missiles that flew 500 km before splashing into the sea. The move was presumably an answer to the joint annual military drills conducted by the US and South Korea, which Pyongyang denounce as 'war games'.

Earlier this week, Kim had said North Korea has miniaturized nuclear warheads that can be mounted on ballistic missiles, following up with a threat earlier that it will launch pre-emptive nuclear strikes on the US and North Korea.

North Korea conducted its fourth nuclear test in January saying it successfully tested a hydrogen bomb. It was followed by a rocket launch, which the US and South Korea believe was a crucial stage in Pyongyang's attempt to developed an intern-continental ballistic missile (ICBM).

The United Nations Security Council unanimously adopted a resolution that vastly expands sanctions on North Korea after its nuclear and missile tests.

Under the tough new sanctions North Korean ships suspected of carrying illegal goods will be banned from ports across the world, while 16 new individuals and 12 organisations from North Korea have been blacklisted.